Method of growing electrical conductors

ABSTRACT

A method for forming a conductive thin film includes depositing a metal oxide thin film on a substrate by an atomic layer deposition (ALD) process. The method further includes at least partially reducing the metal oxide thin film by exposing the metal oxide thin film to a reducing agent, thereby forming a seed layer. In one arrangement, the reducing agent comprises one or more organic compounds that contain at least one functional group selected from the group consisting of —OH, —CHO, and —COOH. In another arrangement, the reducing agent comprises an electric current.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/039,689, filed Feb. 28, 2008, now U.S. Pat. No. 7,955,070,issued Jun. 7, 2011, which is a continuation of U.S. patent applicationSer. No. 10/394,430, filed Mar. 20, 2003, issued Feb. 24, 2009 as U.S.Pat. No. 7,494,927, which is a continuation-in-part of U.S. patentapplication Ser. No. 10/300,169, filed Nov. 19, 2002, issued May 3, 2005as U.S. Pat. No. 6,887,795, which is a continuation of U.S. patentapplication Ser. No. 09/858,820, filed May 15, 2001, issued Nov. 19,2002 as U.S. Pat. No. 6,482,740 B2, which claims the benefit under 35U.S.C. §119(a) of Finnish Patent Application No. 20001163, filed May 15,2000, all of which are incorporated in their entireties by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the manufacturing ofintegrated circuits (ICs), and particularly to the seed layers indamascene and dual damascene processes, gate metals of thin filmtransistors and capacitor electrodes in ICs.

More particularly the present invention relates to a method ofdepositing seed layers for a damascene and dual damascene structures,gate metals of thin film transistors and capacitor electrodes in ICs byan atomic layer deposition (ALD) method.

2. Description of the Related Art

The atomic layer deposition (ALD) method of depositing thin films hasseveral attractive features including excellent step coverage, even onlarge areas, and a dense and pinhole-free structure. Therefore, it is ofgreat interest to apply ALD to the deposition of metallization layers ofadvanced integrated circuits (ICs), where the continuously increasingpacking density and aspect ratio set higher and higher demands upon themetallization layers. Applications where high quality metallization isparticularly needed are dual damascene structures, gates in transistorsand capacitors in ICs. However, due to the fact that ALD is based onsequential self-saturating surface reactions of source chemicalcompounds, depositing high quality elemental metal thin films by ALD isvery difficult.

In ALD, the source chemical molecules chemisorb on the substrate viaactive sites on the substrate surface. Typical active sites for metalsource chemicals are —OH, —NH₂ and —NH groups. Metal-oxygen-metalbridges on the surface may also act as active sites. When a metal sourcechemical molecule reacts with the active site, a strong bond is formedbetween the surface and the ligand of the source chemical molecule issimultaneously released as a by-product.

In ALD, films grow with a constant growth rate. Each deposition cycleproduces one molecular layer of the deposited material on the substratesurface. Usually the growth rate is well below one molecular layer/cyclebecause the adsorbed source chemical molecules may be bulky or becausesubstrate temperature affects the number of active sites (e.g. —OHgroups) on the surface. It is well known that metal oxide thin filmsproduced by ALD are uniform, have excellent adhesion and thus are firmlybonded to the substrate surface.

Experiments have revealed a drawback of the growth of metal thin filmsby an ALD type method. In the case of metal deposition it is difficultto attach source chemical molecules to the surface because essentiallyno active sites exist on the surface. The metal film grown is oftennon-uniform over an area of the substrate and it is easily peeled offfrom the surface, which indicates very poor adhesion of the film to thesubstrate.

Several attempts have been made to produce metal thin films by ALD typemethods. Reproducibility of such an ALD metal growth process hastraditionally been poor and the reactions do not take place at all oninsulating surfaces like silicon oxide. There are publications about theALD deposition of Cu metal by pulsing a copper compound, e.g. Cu(thd)₂,on a surface and then reducing the Cu(thd)₂ molecules bound to thesurface into Cu with H₂.

R. Solanki et al. (Electrochemical and Solid-State Letters 3 (2000)479-480) have deposited copper seed layers by ALD. They deposited copperdirectly from alternate pulses ofbis(1,1,1,5,5,5-hexafluoroacetylacetonato)copper(II)hydrate and eithermethanol, ethanol or formalin, i.e. a water solution of formaldehyde.The total pulsing cycle time was 64 s, i.e. slightly over one minute.Although the growth rate was not mentioned in the publication, a typicalgrowth rate of a thin film made by ALD from metal β-diketonates is 0.03nm/cycle due to the steric hindrance of the source chemical molecules.Thus, the deposition time for a 10-nm copper seed layer would be over 5hours, which is uneconomical for wafer processing. A required minimumthroughput of a wafer reactor is 10-12 wafers/hour. It is to be notedthat according to Strem Chemicals, Inc. the decomposition temperature ofthe copper compound used by R. Solanki et al. is 220° C. R. Solanki etal. noticed copper film growth when the substrate temperature was230-300° C. Therefore, partial thermal decomposition of copper sourcecompound on substrate surface is probable.

One of the most advanced IC structures is the dual damascene structurewhich consists of a silicon substrate with transistors (source, gate anddrain). Several electrically conducting layers are needed in thestructure. The first metallization level is done with tungsten plugs andaluminium interconnects to prevent the contamination of the gate withcopper. The remainder of the metallization levels are made of copper.

There are several ways of making dual damascene structures. An exampleof the process steps of a dual damascene process is described below.

Step 1. A silicon nitride etch stop is grown on the previousmetallization surface.

Step 2. A via level dielectric is deposited.

Step 3. Another silicon nitride etch stop is deposited.

Step 4. A trench level dielectric is deposited. SiO₂ has been favored asthe dielectric material. Low-k materials such as nitrided silicon oxideand polymers have been experimented with as alternative dielectricmaterials.

Step 5. Patterning of dielectric by photolithography.

a. A resist layer is deposited on dielectrics surface.

b. The resist layer is patterned and the resist is removed from the viaareas.

c. Dielectrics are etched from the via areas with directional plasma.Etching terminates at the silicon nitride surface.

d. Resist is stripped from the surface.

Step 6. Patterning of the etch stop layer by photolithography.

e. A second resist layer is deposited on the surface.

f. The resist layer is patterned and it is removed from the trenchareas.

g. Silicon nitride is removed with a short plasma nitride etch from thebottom of the holes that were made with the first plasma oxide etch.

h. The second plasma oxide etch removes silicon dioxide from the exposedvia and trench areas until the first silicon nitride etch stop isreached.

i. The first silicon nitride etch stop is removed from the via bottomand the second silicon nitride etch stop from the trench bottom with ashort plasma nitride etch.

j. The resist is stripped from the substrate.

Step 7. A diffusion barrier layer is grown on all exposed surfaces.

Step 8. A seed layer for copper deposition is grown with CVD or PVD onthe diffusion barrier layer.

Step 9. Vias and trenches are filled with copper by an electroplatingprocess.

Step 10. The substrate surface is planarized with chemical mechanicalpolishing (CMP). The surface is polished until copper and a barrierlayer are left only in trenches and vias.

Step 11. The surface is capped with a silicon nitride etch stop layer.

Step 12. The metallization process is then repeated for all theremaining metallization levels.

Alternatives for copper electroplating (Step 9) are electroless plating,physical vapor deposition (PVD) and chemical vapor deposition (CVD). Aseed layer (c.f. Step 8) is typically needed for electroplatingprocesses. Traditionally such a seed layer is deposited by chemicalvapor deposition (CVD) or physical vapor deposition (PVD). In theelectroplating process the substrate having an electrically conductiveseed layer is immersed in a metal compound solution. The electricallyconductive surface of the substrate is connected to an external DC powersupply. A current passes through the substrate surface into the solutionand metal is deposited on the substrate. The seed layer has highconductivity and it acts as a conduction and nucleation layer for theelectroplating process. One can envision a seed layer that acts as anucleation layer also for the CVD process. The seed layer carriescurrent from the edge of the wafer to the center of the wafer and fromthe top surface of the wafer into the bottom of vias and trenches. Auniform and continuous seed layer is necessary to get uniformelectroplated copper. Electrical contact is made to the seed layer. Thequantity of the deposited metal is directly proportional to the localcurrent density on the substrate.

The benefits of copper compared to aluminum are lower resistivity andbetter resistance to electromigration. Furthermore, since tighterpacking density can be obtained with copper, fewer metallization levelsare needed and the manufacturing costs are lower than with aluminum.With increasing aspect ratio it is becoming difficult to get sufficientstep coverage for the seed layer with the state of the art technology.

In dynamic random access memories (DRAM), capacitors store data bits inthe form of electrical charge. These memory capacitors must be rechargedfrequently due to the leaking of electrons. The simplest capacitorconsists of two parallel metallic plates separated with a dielectricmaterial. The capacity (C) of this plate capacitor depends according toequation (I) on the area (A) of the metallic plate, the distance (d)between the metallic plates and the dielectric constant (k) of thedielectric material. ∈₀ is the permittivity of space.C=k∈ ₀ A/d  (I)

Cylindrical capacitors are often used. The conductors are arrangedcoaxially. The charge resides on the inner wall of the outer conductorand on the outer surface of the inner conductor. In this case thecapacitance (C) depends on the radius of the outer surface of the innerconductor (a), radius of the inner surface of the outer conductor (b),length of the cylinder (1) and dielectric constant (k) of the dielectricmaterial between the conductors as shown in equation (II).C=2πk∈ ₀ l/ln(b/a)  (II)

The feature sizes in DRAMs are decreasing continuously. The capacitorsmust be made smaller in successive DRAM generation. In order to savesurface area, planar capacitors are being replaced with vertical coaxialcapacitors that may have aggressive aspect ratios. Decreasing the chargestoring area means that the distance between the conductors must bedecreased and/or the dielectric constant of the dielectric must beincreased in order to keep the capacity sufficient. Decreasing thedistance between the conductors causes voltage breakdown when theinsulator thickness is too thin to hold the voltage.

Using high-k dielectrics, such as TiO₂ and Ta₂O₅, resolves the abovedescribed problem related to decreasing feature size. However, high-kdielectrics create new problems, since they donate oxygen to theconductor and thus the capacitor properties deteriorate. Therefore,inert metals, such as platinum group metals, or conductive metal oxides,such as RuO₂, must be used adjacent to the high-k metal oxides. But itis difficult to deposit thin films with good step coverage on newcapacitor structures with small feature size and aggressive aspectratio. As a conclusion, there is an increasing need for a method ofproducing conductive thin films with good step coverage and excellentthin properties such as adhesion to the substrate.

S.-J. Won et al. have presented a metal-insulator-metal (MIM) capacitorstructure for giga-bit DRAMs (Technical Digest of the 2000 InternationalElectron Devices Meeting (IEDM), San Francisco, Calif., Dec. 10-13,2000). They used Ta₂O₅ as an insulator while the electrodes consisted ofruthenium which was deposited by CVD from Ru(EtCp)₂ and gaseous oxygenat 300-400° C. Problems related to the method included poor stepcoverage and reaction speed sensitivity. When the nodes were made with0.08 μm design rules, the step coverage dropped to 60%. The reaction ofRu(EtCp)₂ with O₂ was adversely affected by the partial pressures of thesaid compounds.

N⁺ or p⁺ doped polycrystalline silicon has been used as a gate electrodefor transistors. However, several problems are associated with the useof poly-Si gate electrodes. In the case of boron doped p⁺ poly-Si, thediffusion of boron through the gate SiO₂ destroys the electricalproperties of the transistor. Poly-Si is thermodynamically unstableagainst high dielectric constant materials at high processingtemperatures. In addition, poly-Si has rather high resistivity comparedto metals. There is a tendency to replace the SiO₂ gate oxide with ahigh dielectric constant metal oxide. A metal with appropriate workfunction would enable the tailoring of the CMOS threshold voltage.Refractory metals have been suggested for gate metals but the stabilityof the metal-gate oxide interface has been an issue. Platinum groupmetals are potential candidates for gate metals due to their inertnature. However, appropriate methods of depositing high-quality platinumgroup metal thin films for gate electrode applications have not yet beendeveloped.

M. Utriainen et al. have demonstrated (Appl. Surf. Sci. 157 (2000) pp.151-158) that ALD grown metal oxides can be used as interconnects in ICsafter reducing the metal oxides into metals. They studied the direct ALDdeposition of Cu, Ni and Pt and the indirect Ni growth method viareduction of NiO. However, they had problems with the quality of thenickel film: pinholes were formed on the thin films during the reductionof NiO with hydrogen gas.

SUMMARY OF THE INVENTION

Embodiments described herein provide methods of producing high qualityconductive thin films with excellent step coverage, uniform thicknessover a large area and excellent adhesion properties. The thin films maybe used, for example, as seed layers for the electrodeposition of metallayers, and gate metals in thin film transistors and capacitors ofadvanced high-density integrated circuits.

The present method is applicable to the manufacture of conductive thinfilms, preferably comprising one or more of the following elements:rhenium, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium,platinum, copper, silver and gold.

According to one embodiment described herein, a method of forming aconductive thin film comprises depositing a metal oxide thin film on asubstrate by an atomic layer deposition (ALD) process. The methodfurther comprises at least partially reducing the metal oxide thin filmby exposing the metal oxide thin film to an electric current, therebyforming a seed layer. In certain embodiments, the reduction of the metaloxide thin film essentially converts the metal oxide into an elementalmetal seed layer that has sufficient conductivity to be used forsubsequent electrochemical deposition.

According to another embodiment described herein, a method of producinga conductive thin film comprises the steps of (A) placing a substrate ina chamber and (B) exposing the substrate to a vapor phase firstreactant. The first reactant adsorbs no more than a monolayer of metalspecies on the substrate. The method further comprises (C) removingexcess first reactant from the chamber and (D) exposing the substrate toa second vapor phase reactant comprising a compound that is capable ofoxidizing the adsorbed metal species on the substrate into metal oxide.The method further comprises (E) removing excess second reactant fromthe chamber and (F) repeating the above steps B through E at least threetimes to form a metal oxide film. The method further comprises (G)following step F, exposing the substrate to an electric current toreduce the metal oxide to metal.

According to still another embodiment described herein, a method ofproducing a conductive thin film comprises depositing a metal oxide thinfilm of at least 0.6 nm thickness on a substrate. The method furthercomprises reducing said metal oxide thin film to metal by exposing themetal oxide thin film to an electric current.

According to yet another embodiment described herein, a method ofproducing a conductive thin film comprises depositing a metal oxide thinfilm on a substrate by an atomic layer deposition (ALD) process. Themethod further comprises at least partially reducing the metal oxidethin film to elemental metal. In certain embodiments, the metal oxidethin film is at least partially reduced by exposing the metal oxide thinfilm to an electric current. In other embodiments, the metal oxide thinfilm is at least partially reduced by exposing the metal oxide thin filmto one or more organic compounds that contain at least one functionalgroup selected from the group consisting of —OH, —CHO, and —COOH.

According to another embodiment described herein, a method of producinga conductive thin film comprises depositing a metal oxide thin film on asubstrate by an atomic layer deposition (ALD) process. The methodfurther comprises at least partially reducing the metal oxide thin filmto elemental metal in an electrochemical deposition tool.

Certain embodiments described herein are especially beneficial formaking electrically conductive layers in structures that have highaspect ratios, like vias, trenches, local high elevation areas and othersimilar surface structures that make the surface rough and thin filmprocessing complicated by conventional CVD and PVD methods. An ALD metaloxide process combined with a reduction step provides excellent stepcoverage of conductive thin films on all surface formations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the dual damascene structure.

FIG. 2 is a schematic view of a capacitor structure.

FIG. 3 is a schematic view of an NMOS transistor suitable for CMOSstructures.

FIG. 4 depicts the electrical resistance of reduced copper samples.

FIG. 5 schematically illustrates a first cluster tool in accordance withembodiments of the present invention.

FIG. 6 schematically illustrates a second cluster tool in accordancewith embodiments of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The dual damascene structure shown in FIG. 1 consists of a previousmetallization layer 2 (e.g., Cu), an insulating layer 4 (e.g., SiO₂), avia etch stop 6 (e.g., Si₃N₄), a via level insulator 8 (e.g., SiO₂), atrench etch stop 10 (e.g., Si₃N₄), a trench level insulator 12 (e.g.,SiO₂), a diffusion barrier 14 (e.g., TaN), a seed layer 16 and avia/trench fill metal 18 (e.g., Cu).

The capacitor structure shown in FIG. 2 consists of a contact plug 30(e.g., tungsten (W) or polysilicon), an insulator 32, an optionaldiffusion barrier 34 (e.g., TiN), a lower electrode 36 (e.g., Ru, Pt, orRuO₂), a high-k dielectric film 38 (e.g., barium strontium titanate(BST)), and an upper electrode 40 (e.g., Ru or Pt).

The partial transistor structure shown in FIG. 3 consists of a substrate60, an n-type well 62, a p-type diffusion region 64 (right drain, leftsource), a shallow trench isolation oxide 66, a gate dielectric 68, anoptional barrier layer 70, a gate metal 72, a gate isolation spacers 74,and contact areas for tungsten plugs 76. The contact areas are dottedbecause they are not in the same vertical plane with the other numberedparts. A CMOS structure contains both PMOS and NMOS transistors. Thecontact areas against P-type semiconductor can be made of, e.g., Ni, andRuO. The contact areas against N-type semiconductor can be made of,e.g., Ru. Platinum can also be applied under W plugs. The choice of themetal or electrically conductive metal compound depends on the workfunction of the underlying layer and the reactivity of the surroundingmaterials with the said metal or electrically conductive metal compound.

FIG. 4 shows the resistance values of reduced copper samples. The valuesare the average of 10 measurements from each sample. ECD stands forcopper metal deposited on silicon. ECD-1 and ECD-2 indicate pure ECDcopper metals without any additional treatments. O-ECD-1 and O-ECD-2indicate samples that have a copper oxide coating (about 350 nm). Therest of the samples had a copper oxide coating before the reductionexperiments. N₂ is nitrogen gas with a claimed purity of 99.9999%, MeOHis methanol, EtOH is ethanol, 2-PrOH is isopropanol, tert-BuOH istert-butanol, PrCHO is butyraldehyde, Me₂CO is acetone, HCOOH is formicacid, CH₃COOH is acetic acid, and H₂ is hydrogen. The number after thereagent name is the reaction temperature in degrees C. The number inparentheses is the reaction time in minutes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A layer of a low volatility oxide of a metal is grown on a substrate.Preferably the metal oxide is grown on the substrate according to theprinciples of an ALD process, and the following disclosure is directedto this embodiment. However, the skilled artisan will recognize thatother methods of depositing a metal oxide thin film may be used in themethods of the invention. After the ALD process, the thin film consistsessentially of a metal oxide or a mixture of metal oxides. The metaloxides are at least partially converted into a metal in a separateprocess step to increase the conductivity of the deposited oxide thinfilm. The conversion step can be done with any reducing agent capable offorming a stronger bond to oxygen than the metal to be reduced.Preferably, the reducing agent is in the gaseous phase. However, in thecase of silver and gold oxides, the conversion step can be done simplyby heating to decompose the oxide into metal and oxygen. In still otherembodiments described herein, the reducing agent comprises an electriccurrent applied to the metal oxide.

The following low volatility metal oxides and mixtures and nanolaminatesof the following metal oxides are examples of compounds that aresuitable for conversion into a conductive form by the method of certainembodiments: ReO₂, Re₂O₅, ReO₃, RuO₂, OsO₂, CoO, CO₃O₄, Rh₂O₃, RhO₂,IrO₂, NiO, PdO, PtO₂, Cu₂O, CuO, Ag₂O (decomposes thermally attemperatures above 230° C.), Au₂O₃ (decomposes thermally at temperaturesabove 160° C.). However, a person skilled in the art will understandthat embodiments described herein are not limited to these metal oxides,in part because the stoichiometry may vary in metal oxide films. Inaddition, the following high-volatility metal oxides exist: Re₂O₇, RuO₄and OsO₄.

A metal oxide layer is preferably produced by the ALD process. A typical

ALD process comprises the following steps:

1. placing a substrate into a reaction chamber;

2. feeding into the reaction chamber and contacting the substrate with apulse of at least one first source chemical, preferably in the vaporphase, comprising a compound capable of adsorbing no more than amonolayer of metal species on the substrate;

3. removing gases from the chamber;

4. feeding into the reaction chamber and contacting the substrate with apulse of at least one second source chemical, preferably in the vaporphase, comprising a compound capable of oxidizing the metal species onthe substrate into a metal oxide, and

4. removing gases from the chamber; and

5. repeating steps 2 through 4 until a desired thickness of the growingthin film is reached.

According to the ALD principles, the previous reactant (i.e. previouslypulsed source chemical) and the gaseous by-products of the surfacereaction are removed from the reaction chamber before the next pulse ofa reactant is introduced into the reaction chamber. The reactants andthe by-products can be removed from the reaction chamber by pumping downthe chamber to a higher vacuum by a vacuum pump, by purging the chamberwith an inert gas pulse, or by a combination of the two.

In the methods of certain embodiments described herein, the ALD cycledescribed above is preferably repeated at least 3 times, more preferablyat least 10 times. A metal oxide thin film of at least 0.6 nm ispreferably formed on the substrate.

“Metal species” in the context of the present application means aseparate molecule, atom or ion comprising one or more metal atoms.

According to one embodiment (FIG. 1), a substrate with open trenches andvias is provided into an ALD reaction chamber. A diffusion barrier layer14 is on the surfaces of the substrate. The pressure of the reactionchamber is adjusted to about 5-10 mbar with a vacuum pump and flowingnitrogen gas. A metal oxide thin film is grown on the diffusion barrier14 from alternate pulses of a metal source chemical and oxygen sourcechemical. Surplus source chemical and reaction by-products areessentially removed from the reaction chamber after each source chemicalpulse before the next source chemical pulse is introduced into thereaction chamber and contacted with the substrate surface. The pulsingcycle is repeated until the thickness of the metal oxide film issufficient for seed layer purposes. The metal oxide film is reduced intoa metal layer and used as a seed layer 16 for an electroplating process.

According to a second embodiment (FIG. 2), a substrate is provided intoa reaction chamber of an ALD reactor. The substrate is heated to adeposition temperature of selected metal oxide. Alternate gas phasepulses of a metal source chemical and an oxygen source chemical areintroduced into the reaction chamber and contacted with the substratesurface. A metal oxide film is grown on the surface. The metal oxide isused as the first electrode 36 of a capacitor or converted intocorresponding metal and used as the first electrode 36 of the capacitor.Then a thin film of a high-k dielectric material 38 is grown on thefirst electrode 36. The high-k layer 38 is optionally annealed. A metaloxide thin film is grown by ALD on the high-k layer. The metal oxidefilm is converted into corresponding metal and used as the secondelectrode 40 of a capacitor. However, the metal oxide thin film can beused as the second electrode of the capacitor if the conductivity of themetal oxide thin film is sufficiently high. In certain embodiments, themetal oxide thin film is used as the second electrode when itsresistivity is preferably less than about 500 μΩ-cm, more preferablyless than about 300 μΩ-cm, and most preferably less than about 100μΩ-cm. An example of a suitable metal oxide is ruthenium dioxide (RuO₂)that has a resistivity of about 35 μΩ-cm.

According to a third embodiment (FIG. 3), a substrate is provided into areaction chamber of an ALD reactor. The surface may be, for example, atransistor gate oxide 68 or doped silicon on source and drain areas 64.The substrate is heated to a deposition temperature. Alternate gas phasepulses of a metal source chemical and an oxygen source chemical areintroduced into the reaction chamber and contacted with the substratesurface. Metal oxide film is grown on the surface. The metal oxide isused as the gate electrode of a transistor as such or converted into thecorresponding metal and used as the gate electrode of a transistor. Themetal is also used as an intermediate layer 76 between silicon andtungsten plugs on the source and the drain areas of the transistor.

According to one embodiment described herein, the metal oxide isruthenium oxide and one deposition cycle of ruthenium oxide consists ofa ruthenium halide pulse, nitrogen purge, a pulse of a reducing agent,nitrogen purge, an oxygen source pulse and nitrogen purge. It is knownthat RuF₆ and RuF₅ can be reduced with iodine into RuF₄ (N. N. Greenwoodand A. Earnshaw in “Chemistry of the Elements”, Pergamon Press, 1984, p.1258). When, e.g., RuF₅ chemisorbs on a surface, Ru should release atleast one F and form a bond to the surface. Equation (III) illustratesthe reaction.RuF₅(g)+—OH(ads.)----->—O—RuF₄(ads.)+HF(g)  (III)

The oxidation state of Ru in RuF₅ is +5, which is too high for theformation of RuO₂. According to one embodiment described herein, Ru(V)compound that is adsorbed on a surface is reduced into Ru(IV) compoundwith gaseous iodine. After the reduction process the oxidation state ofruthenium in the adsorbed Ru compound is +4 and results in the growth ofRuO₂ when a pulse of oxygen source chemical is introduced into thereactor and contacts the surface of the substrate. Equations (IV) and(V) illustrates the reactions:10(—RuF₄)(ads.)+I₂(g)----->10(—RuF₃)(ads.)+2IF₅(g)  (IV)

When an oxygen source chemical is contacted with the surface a reactionaccording to equation (V) takes place:2(—RuF₃)(ads.)+4H₂O(g)----->2[—RuO(—OH)](ads.)+6HF(g)  (V)

Due to equal oxidation states of Ru in the adsorbed Ru source compoundand Ru in the resulting ruthenium oxide, the removal of F ligands fromthe surface in the form of gaseous HF molecules is a straightforwardprocedure.

According to one embodiment described herein, the metal oxide thin filmdeposited by ALD consists of metallic ruthenium and ruthenium oxide. Thegrowth of the ruthenium-rich film is based on reduction-oxidation(redox) reactions on the surface.

Known oxides of ruthenium are RuO₂ and RuO₄. The vapor pressure of RuO₂is negligible at the reactor temperatures used according to certainembodiments described herein. However, RuO₄ has such a high vaporpressure, 760 torr at 40° C., that it can easily be evaporated from anexternal source.

One deposition cycle for the growth of ruthenium-rich thin film consistsof a RuO₄ pulse, a nitrogen purge, a pulse of vaporized organic reducingagent, and a nitrogen purge. Therefore, in the beginning of the growthprocess ruthenium tetra-oxide (RuO₄) is adsorbed on the substratesurface. Adsorbed ruthenium oxide is reduced to metallic ruthenium withan organic reducing agent, ruthenium tetra-oxide is contacted again withthe surface of the substrate when RuO₂ is formed and then RuO₂ is atleast partly reduced with an organic reducing agent to metallicruthenium.

Ruthenium metal is in the oxidation state of 0, while ruthenium has anoxidation state of VIII in RuO₄. On the substrate surface there isruthenium with low oxidation state. High oxidation state rutheniumsource chemical is contacted with the surface. A reduction-oxidation(redox) reaction according to equation (VI) takes place on the substratesurface:Ru(ads.)+RuO₄(g)---->2RuO₂(ads.)  (VI)

In addition, one molecular layer of RuO₄ may adsorb on thelow-volatility RuO₂ surface.

Then the surface is treated with a reducing agent pulse:RuO₂(ads.)+2CH₃CH₂OH----->Ru(ads.)+2CH₃CHO(g)+2H₂O(g)  (VII)

Suitable reducing agents include alcohols, aldehydes, carboxylic acidsand hydrogen radicals.

Ruthenium metal thin film and/or ruthenium oxide thin film producedthereof can be used as a seed layer and/or an electrode.

Deposition of ternary ruthenium compounds is also possible. For example,SrRuO₃, which is suitable electrode material, is grown by ALD byapplying alternate source chemical pulses of a Sr compound, Ru compoundand oxygen compound.

According to one embodiment described herein, OsO₄ is prepared in situwith a highly reactive oxygen compound from low volatility Os metal orosmium oxides such as OsO₂. OsO₄ requires careful handling due to itshigh toxicity and volatility (vapour pressure is 760 torr at 130° C.). Alow-volatility Os source is placed in a zone in front of the substratespace. The source is heated to about 130-150° C. Then, e.g., ozone ispulsed over Os.Os(s)+4O₃(s)----->OsO₄(g)+4O₂(g)  (VII)

Surplus O₃ gas is decomposed into O₂ before the reaction chamber toprotect the OsO₂ layer that is growing on the substrate surface. O₃ iseasily decomposed by heating or with molecular sieves such as zeolites.

When depositing silver and gold oxides by ALD, special attention has topaid to the selection of growth temperatures, since Ag₂O decomposes intoAg and O₂ at temperatures above 230° C. and Au₂O₃ decomposes into Au andO₂ at temperatures above 160° C. Therefore, the deposition temperatureof silver oxide is preferably kept below 230° C. and the depositiontemperature of gold oxide is preferably below 160° C.

The Source Chemicals

The ALD source chemicals must have sufficient volatility at the sourcetemperature. The vapor pressure of the source chemical should be atleast about 0.02 mbar at the source temperature to enable reasonablyshort pulse times for saturating the substrate surfaces. The metalsource chemicals must be thermally stable at the deposition temperatureto prevent particle formation in the gas-phase of the reaction chamber.

Suitable metal source compounds are sold, for example, by StremChemicals, Inc. (7 Mulliken Way, Dexter Industrial Park, Newburyport,Mass., USA) and Tri Chemical Laboratory, Inc. (969 Helena Drive,Sunnyvale, Calif., USA).

Low oxidation state rhenium oxide can be grown by ALD for example fromfollowing rhenium compounds:

-   rhenium(VII) oxide (Re₂O₇), rhenium pentacarbonyl chloride    (Re(CO)₅Cl), methyltrioxorhenium(VII) (CH₃ReO₃),    cyclopentadienylrhenium tricarbonyl ((C₅H₅)Re(CO)₃),    pentamethylcyclopentadienylrhenium tricarbonyl ([(CH₃)₅C₅]Re(CO)₃),    and i-propylcyclopentadienylrhenium tricarbonyl ((C₃H₇)C₅H₄Re(CO)₃).

Low oxidation state ruthenium oxide can be grown by ALD for example fromfollowing ruthenium compounds:

-   ruthenium(VIII)oxide (RuO₄), bis(cyclopentadienyl)ruthenium    ((C₅H₅)₂Ru), bis(pentamethylcyclopentadienyl)ruthenium    ([(CH₃)₅C₅]₂Ru), cyclopentadienylruthenium dicarbonyl    ((C₅H₅)₂Ru(CO)₂),    bis(2,2,6,6-tetramethyl-3,5-heptanedionato)(1,5-cyclooctadiene)ruthenium(II)    (C₁₁H₁₉O₂)₂(C₈H₁₂)Ru, tris(dipivaloylmethanato)ruthenium i.e.    tris(2,2,6,6-tetramethyl-3,5-heptanedionato)ruthenium (Ru(DPM)₃ or    Ru(thd)₃), anhydrous rutheniumnitrate (Ru(NO₃)₃), and anhydrous    ruthenium (III) nitrosyl nitrate (Ru(NO)(NO₃)₃).

Low oxidation state osmium oxide is preferably grown by ALD for examplefrom following osmium compounds:

-   bis(cyclopentadienyl)osmium ((C₅H₅)₂Os),    bis(pentamethylcyclopentadienyl)osmium ([(CH₃)₅C₅]₂Os), and    osmium(VIII)oxide (OsO₄).

Cobalt oxide is preferably grown by ALD for example from followingcobalt compounds:

-   bis(cyclopentadienyl)cobalt(II) ((C₅H₅)₂Co),    bis(methylcyclopentadienyl)cobalt(II) ((CH₃C₅H₄)₂Co),    bis(pentamethylcyclopentadienyl)cobalt(II) ([(CH)₃C₅]₂Co), cobalt    tricarbonyl nitrosyl (Co(CO)₃NO), cyclopentadienylcobalt dicarbonyl    C₅H₅Co(CO)₂, cobalt(III)acetylacetonate (Co(CH₃COCHCOCH₃)₃)    tris(2,2,6,6-tetramethyl-3,5-heptanedionato)cobalt(III) i.e.    tris(dipivaloylmethanato)cobalt (Co(TMHD)₃, or Co(DPM)₃, or    Co(thd)₃, or Co(C₁₁H₁₉O₂)₃), and anhydrous cobalt nitrate    (Co(NO₃)₃), the synthesis of which has been described by R. J. Logan    et al. in J. Chem. Soc., Chem. Commun. (1968) 271.

Rhodium oxide is preferably grown by ALD for example from followingrhodium compounds:

-   2,4-pentanedionatorhodium(I)dicarbonyl (C₅H₇Rh(CO)₂),    tris(2,4-pentanedionato)rhodium rhodium(III)acetylacetonate    (Rh(C₅H₇O₂)₃), and tris(trifluoro-2,4-pentanedionato)rhodium.

Iridium oxide is preferably grown by ALD for example from followingiridium compounds:

-   (methylcyclopentadienyl)(1,5-cyclooctadiene)iridium(I)    ([(CH₃)C₅H₄](C₈H₁₂)Ir) and trisallyliridium ((C₃H₅)₃Ir).

Nickel oxide is preferably grown by ALD for example from followingnickel compounds:

-   nickel carbonyl (Ni(CO)₄),    bis(2,2,6,6-tetramethyl-3,5-heptanedionato)nickel(II) (Ni(DPM)₂, or    Ni(thd)₂, or Ni(C₁₁H₁₉O₂)₂), nickel(II) acetylacetonate, also known    as bis(2,4-pentanedionato)nickel(II), nickel(II)    trifluoroacetylacetonate, nickel(II)hexafluoroacetylacetonate    (Ni(CF₃COCHCOCF₃)₂), nickel(II) dimethylglyoxime (Ni(HC₄H₆N₂O₂)₂),    and tetrakis(trifluorophosphine)nickel(0) (Ni(PF₃)₄).

Palladium oxide is preferably grown by ALD for example from followingpalladium compounds: Pd(thd)₂ andbis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)palladium(Pd(CF₃COCHCOCF₃)₂).

Platinum oxide is preferably grown by ALD for example from followingplatinum compounds:

-   platinum(II)hexafluoroacetylacetonate (Pt(CF₃COCHCOCF₃)₂),    (trimethyl)methylcyclopentadienylplatinum(IV) ((CH₃)₃(CH₃C₅H₄)Pt),    and allylcyclopentadienylplatinum ((C₃H₅)(C₅H₅)Pt).

Copper oxide is preferably grown by ALD for example from the followingcopper compounds and their derivatives:

-   CuCl, CuBr, CuI,    bis(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)copper(II)    (Cu(FOD)₂), bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II)    (Cu(TMHD)₂, or Cu(DPM)₂, or Cu(thd)₂), copper(II)acetylacetonate    (Cu(CH₃COCHCOCH₃)₂), also known as Cu(acac)₂, derivatives of    Cu(acac)₂ such as alkyl derivatives of Cu(acac)₂,    copper(II)trifluoroacetylacetonate (Cu(CF₃COCHCOCH₃)₂),    copper(II)hexafluoroacetylacetonate (Cu(CF₃COCHCOCF₃)₂),    hexafluoroacetylacetonatocopper(I)trimethylphosphine adduct    (Cu(CF₃COCHCOCF₃)P(CH₃)₃), copper(II)dialkylaminoalkoxides such as    copper(II)dimethylaminoethoxide,    cyclopentadienylcopper(I)triethylphosphine ((C₅H₅)Cu:P(C₂H₅)₃),    ethylcyclopentadienylcopper triphenylphosphine adduct    ((C₂H₅C₅H₄)Cu:P(C₆H₅)₃),    hexafluoroacetylacetonatocopper(I)triethylphosphine adduct    ((C₅HF₆O₂)Cu:P(C₂H₅)₃), hexafluoroacetylacetonatocopper(I)2-butyne    adduct ((C₅HF₆O₂)Cu:CH₃CCCH₃),    hexafluoroacetylacetonatocopper(I)1,5-cyclooctadiene adduct    ((C₅HF₆O₂)Cu:C₈H₁₂),    hexafluoropentanedionatocopper(I)vinyltrimethylsilane adduct, and    anhydrous copper nitrate (Cu(NO₃)₂), the synthesis of which has been    described by C. C. Addison et al. (J. Chem. Soc. (1958) pp.    3099-3106).

Silver oxide is preferably grown by ALD for example fromhexafluoroacetylacetonatosilver trimethylphosphine adduct((C₅HF₆O₂)Ag:P(CH₃)₃).

Gold oxide is preferably grown by ALD for example from following goldcompounds:

gold(III)fluoride AuF₃, dimethyl(acetylacetonato)gold(III)((CH₃)₂(C₅H₇O₂)Au), and dimethylhexafluoroacetylacetonatogold((CH₃)₂Au(C₅HF₆O₂)).

The oxygen source material used in the method of certain embodiments isselected from a group of volatile or gaseous compounds that containoxygen and are capable of reacting with an adsorbed metal compound onthe substrate surface, at the deposition conditions, resulting in growthof metal oxide thin film on the substrate surface.

It is to be noted that Re, Ru and Os form highly volatile oxides whenreacting with strong oxidizing agents. It is therefore necessary toexclude strong oxidizing agents from the vicinity of the substrate whengrowing lower oxidation state oxides of Re, Ru and Os.

In the production of a metal oxide thin film on a wafer the oxygensource chemical is selected for example from a group consisting of water(H₂O), hydrogen peroxide (H₂O₂), oxygen (O₂), ozone (O₃), singlet oxygen¹O₂, and oxygen compounds with unpaired electrons, as well as oxygenradicals and OH radicals.

A special group of oxygen source chemicals can be used when the metalsource chemical is an anhydrous metal nitrate. This group of oxygenchemicals consists of an aqueous solution of ammonia (NH₃*H₂O or NH₄OH),an aqueous solution of hydroxylamine (NH₂OH*H₂O), and an aqueoussolution of hydrazine (N₂H₄*H₂O).

Mixtures of at least two oxygen source chemicals can also be used formetal oxide deposition. Especially in case of ozone, the substratesurface may remain too “dry” (i.e. the surface contains too few —OHgroups) and the number of active surface sites (especially —OH groups)will drop below an optimum value. By adding certain amount of watervapor to the ozone pulse, the number of —OH groups on the surface can beincreased and the growth rate of the metal oxide thin film can beimproved. Also water vapor can be pulsed after the O₃ pulse to achievethe same result as with the water-ozone pulse.

Reduction Process

According to certain embodiments described herein, the metal oxide thinfilm that is to be reduced into a metal thin film consists essentiallyof a metal oxide or a mixture of metal oxides. The method of reducingthe metal oxide layer plays a very important role in certain embodimentsdescribed herein. The metal oxide is reduced in certain embodiments bymeans of volatile organic reducing agents, such as organic compoundsthat contain at least one —OH, —CHO and/or —COOH functional group orgaseous mixtures of said organic compounds. Surprisingly, good adhesionof the reduced metal oxide thin film is preserved when the abovementioned organic reducing agents are used. In comparison, the use ofvery strong reducing agents, such as hydrogen plasma, causes damage tothe underlying films and may result in harmful hydride incorporation inthe film at higher temperatures and variable adhesion between films. Inother embodiments, the reducing agent comprises an electric currentapplied to the metal oxide thin film.

Chemical Reducing Agents

In certain embodiments, the conversion step is preferably done with areducing agent capable of forming a stronger bond to the oxygen of themetal oxide layer than the metal in the said oxide layer. In certainembodiments, the reduction can be performed by introducing the reducingagent into the electrochemical deposition (ECD) tool as is used forsubsequent metal deposition. In addition, the chemical reducing agentcan be used in the ECD tool, especially in electroless plating tools. Incertain other embodiments, the reducing agent is in gaseous form. Thegaseous reducing agent is capable of taking away the oxygen that wasbound to the metal oxide and thus an elemental metal is left on thesubstrate surface. For example, primary alcohols react into aldehydesand a water molecule is released as a byproduct. Aldehydes take oneoxygen atom and react into carboxylic acids, without the formation of awater molecule.

According to one embodiment described herein, reducing agents thatcomprise relatively bulky molecules (alcohols, aldehydes and carboxylicacids) are used. Bulky source chemical molecules do not easily diffuseinside the metal oxide film. Thus the reduction reaction takes placeonly at the surface of the metal oxide layer. During the reductionprocess, oxygen ions diffuse towards the surface where oxygen isdepleted by the reducing chemicals. Gaseous by-products are thus notformed inside the film, but only on the surface. The structuralintegrity of the forming metal film is thereby preserved and theformation of pinholes on the film will be avoided.

The reduction process of such embodiments is preferably carried out in areaction space that enables controlled temperature, pressure and gasflow conditions. The organic reducing agent is preferably vaporized andfed to the reaction space, optionally with the aid of an inert carriergas, such as nitrogen. The reducing agent is contacted with thesubstrate, whereby the metal oxide layer is reduced at least partly tometal and the reducing agent is oxidized. Typically, the reaction spaceis then purged with an inert carrier gas to remove the unreacted organicreducing agent and reaction products.

The reduction process according to certain embodiments is preferablycarried out at low temperatures. Theoretically, the reactions betweenoxide(s) and the reducing agents used in the process of certainembodiments are favorable in a wide temperature range, even as low asroom temperature. Kinetic factors and the diffusion rate of oxygen tothe thin film surface set a lower limit on the actual processtemperatures that can be applied successfully. The temperature in thereaction space is preferably in the range of 200° C. to 400° C., morepreferably 300° C. to 400° C. and even more preferably 310° C. to 390°C. It is to be noted that in case of very thin metal oxide films, thereduction temperature can be even lower than 250° C. If the depositionand reduction temperature are very low, thus causing a slow reductionreaction or slow diffusion of oxygen through the metal oxide layer, thedeposition of the metal film can be divided into at least two parts tospeed up the total processing time. One layer of the metal oxide,preferably comprising more than one monolayer, is deposited by ALD, thenreduced into a metal layer, another layer, preferably comprising morethan one monolayer of the metal oxide, is deposited by ALD, then reducedinto a metal layer, et cetera, until a metal film of desired thicknessis obtained.

In case of electrochemical reduction, the reduction temperature ispreferably between about 0° C. and about 100° C., more preferablybetween about 20° C. and about 80° C. and most preferably between about50° C. and about 60° C.

The pressure in the reaction space is preferably 0.01 to 20 mbar, morepreferably 1 to 10 mbar.

The processing time varies according to the thickness of the layer to bereduced. A layer of copper oxide having a thickness up to 300-400 nm canbe reduced in approximately 3 to 5 minutes. For layers having athickness of approximately 0.1-10 nm, the processing time is in theorder of seconds. Preferably the layer to be reduced has a thickness ofat least 0.6 nm.

The organic compounds used in the reduction according to certainembodiments described herein preferably have at least one functionalgroup selected from the group consisting of alcohol (—OH), aldehyde(—CHO), and carboxylic acid (—COOH).

Suitable reducing agents containing at least one alcohol group arepreferably selected from the group consisting of:

-   -   primary alcohols which have an —OH group attached to a CH₃ group        (CH₃OH) or to a carbon atom which is bonded to one other carbon        atom, in particular primary alcohols according to the general        formula (I)        R¹—OH  (I)    -   wherein R¹ is a linear or branched C₁-C₂₀ alkyl or alkenyl        group, preferably methyl, ethyl, propyl, butyl, pentyl or hexyl,    -   examples of preferred primary alcohols being methanol (CH₃OH),        ethanol (CH₃CH₂OH), propanol (CH₃CH₂CH₂OH), butanol        (CH₃CH₂CH₂CH₂OH), 2-methyl propanol ((CH₃)₂CHCH₂OH) and 2-methyl        butanol (CH₃CH₂CH(CH₃)CH₂OH),    -   secondary alcohols which have an —OH group attached to a carbon        atom which is bonded to two other carbon atoms, in particular        secondary alcohols according to formula (II)

-   -   wherein each R¹ is selected independently from the group of        linear or branched C₁-C₂₀ alkyl and alkenyl groups, preferably        methyl, ethyl, propyl, butyl, pentyl and hexyl, examples of        suitable secondary alcohols being 2-propanol ((CH₃)₂CHOH) and        2-butanol (CH₃CH(OH)CH₂CH₃),    -   tertiary alcohols which have an —OH group attached to a carbon        atom which is bonded to three other carbon atoms, in particular        tertiary alcohols according to the general formula (III)

-   -   wherein each R¹ is selected independently from the group of        linear or branched C₁-C₂₀ alkyl and alkenyl groups, preferably        methyl, ethyl, propyl, butyl, pentyl and hexyl, an example of        suitable tertiary alcohol being tert-butanol ((CH₃)₃COH),    -   polyhydroxy alcohols, such as diols and triols, which can have        primary, secondary and/or tertiary alcohol groups as presented        above, for example ethylene glycol (HOC₂CH₂OH) and glycerol        (HOCH₂CH(OH)CH₂OH),    -   cyclic alcohols which have an —OH group attached to at least one        carbon atom which is part of a ring of 1-10, typically 5-6        carbon atoms,    -   aromatic alcohols having at least one —OH group attached either        to the benzene ring or to a carbon atom in a side-chain, such as        benzyl alcohol (C₆H₅CH₂OH), o-, p- and m-cresol and resorcinol,    -   halogenated alcohols, preferably having the general formula (IV)        CH_(n)X_(3-n)—R²—OH  (IV)    -   wherein X is F, Cl, Br or I, preferably F or Cl,        -   n is an integer from 0 to 2, and            -   R² is selected from the group of linear or branched                C₁-C₂₀ alkylene and alkenylene groups, preferably                methylene, ethylene, trimethylene, tetramethylene,                pentamethylene and hexamethylene, and in particular                methylene and ethylene,    -   an example of a suitable compound is 2,2,2-trifluoroethanol        (CF₃CH₂OH), and    -   other derivatives of alcohols, for example amines, such as        methyl ethanolamine (CH₃NHCH₂CH₂OH).

Suitable reducing agents containing at least one —CHO group arepreferably selected from the group consisting of

-   -   compounds having the general formula (V)        R³—CHO  (V)    -   wherein R³ is hydrogen or linear or branched C₁-C₂₀ alkyl or        alkenyl group, preferably methyl, ethyl, propyl, butyl, pentyl        or hexyl, in particular methyl or ethyl, examples of suitable        compounds according to formula (V) are formaldehyde (HCHO),        asetaldehyde (CH₃CHO) and butyraldehyde (CH₃CH₂CH₂HO),    -   alkanedial compounds having the general formula (VI)        OHC—R⁴—CHO  (VI)    -   wherein R⁴ is a linear or branched C₁-C₂₀ saturated or        unsaturated hydrocarbon, but it is also possible that R⁴ is        zero, i.e., the aldehyde groups are bonded to each other,    -   halogenated aldehydes, and    -   other derivatives of aldehydes.

Suitable reducing agents containing at least one —COOH group arepreferably selected from the group consisting of

-   -   compounds having the general formula (VII)        R⁵—COOH  (VII)    -   wherein R⁵ is hydrogen or linear or branched C₁-C₂₀ alkyl or        alkenyl group, preferably methyl, ethyl, propyl, butyl, pentyl        or hexyl, in particular methyl or ethyl, examples of suitable        compounds according to formula (VI) being formic acid (HCOOH)        and acetic acid (CH₃COOH),    -   polycarboxylic acids,    -   halogenated carboxylic acids, and    -   other derivatives of carboxylic acids.

Reactors used for deposition of thin films by ALD and/or CVD arepreferably used in the methods of certain embodiments described herein.However, the deposition of the metal oxide thin film and the reductionstep in embodiments using chemical reduction agents are preferablycarried out sequentially in one reactor. The reduction process can alsobe done in a cluster tool where the substrate arrives from a previousprocess step, the substrate is treated with the reducing agent andfinally transported to the following process step. In a cluster tool thereaction space temperature can be kept constant, which improves thethroughput when compared to a reactor which is heated to the processtemperature before each run.

Electric Current as Reducing Agent

In certain embodiments, the deposited metal oxide thin film is reducedby exposing it to an electric current. In certain such embodiments, theelectric current is generated by placing the substrate in anelectrochemical deposition (ECD) or electrochemical mechanicaldeposition (ECMD) tool, such as the NuTool 2000™ tool available fromNuTool, Milpitas, Calif., USA. Reduction of the metal oxide thin film toa metal thin film can create a seed layer for subsequent layer formationusing electrochemical deposition or electroless deposition. In certainembodiments, the reduction can be performed in the same ECD tool as isused for subsequent metal deposition.

In an exemplary embodiment, a conformal copper oxide (CuO) film isdeposited by ALD on a barrier film. Suitable barrier films for copperinclude, but are not limited to, TiN, TaN and WNC. The CuO film has athickness preferably at least 0.6 nanometers, more preferably betweenapproximately 1 nanometer and approximately 20 nanometers, and mostpreferably between approximately 1 nanometer and approximately 3nanometers.

Electric current can be used as the reducing agent for the CuO film byusing the following reactions:CuO(s)+H₂O+2e ⁻-->Cu(s)+2OH⁻2OH⁻-->2OH+2e ⁻2OH⁻-->H₂O+1/2O₂(g).These reactions can be used to normally reduce native CuO in ECD tools,and can also be used to reduce the ALD-generated CuO film which isdeposited on a conductive barrier film layer. The resultant Cu film canthen be used as a seed layer in an ECD tool for subsequent metal layerformation. In certain embodiments, the seed layer has a resistivity ofpreferably between about 1 μΩ-cm and about 30 μΩ-cm, more preferablybetween about 1.67 μΩ-cm and about 10 μΩ-cm, and most preferably betweenabout 1.7 μΩ-cm and about 3 μΩ-cm. Resulting structures can be used inmicrochip metallization such as single and dual damascene processes.

In certain embodiments, the electrolyte solution preferably compriseswater-soluble metal hydroxide such as alkali metal hydroxide, e.g.sodium hydroxide, dissolved in purified water. Metal hydroxide increasesthe pH of the solution so that copper oxide does not dissolve from thesurface into the solution. Hydroxyl ions act as charge and oxygencarriers in the solution. In certain embodiments, the electric currentis applied at temperatures preferably between about 0° C. and about 100°C., more preferably between about 20° C. and about 80° C., and mostpreferably between about 50° C. and about 60° C. In certain embodiments,the electric current is applied for a time period of preferably betweenabout 1 second and about 3600 seconds, more preferably between about 30seconds and about 1000 seconds.

Preferably, the subsequent metal layer formation is performed using thesame ECD tool as is used for reducing the CuO film. An exemplary ECDtool comprises two or more ECD modules so that one ECD module containsalkaline solution for the copper oxide reduction and one ECD modulecontains acidic solution for the copper metal deposition. An example ofsuch a tool is LuminaCu™ system available from NuTool, Milpitas, Calif.,USA. In addition, prior to the formation of the subsequent metal layerusing ECD, the resulting Cu seed layer can be repaired using known seedrepair technologies, such as electroless deposition processes (see,e.g., Peter Singer, “Progress in Copper: A Look Ahead,” SemiconductorInternational, May 1, 2002, the disclosure of which is incorporated inits entirety by reference herein). The basics of electroless deposition,also known as electroless plating, have been presented by G. Mallory andJ. Hadju in “Electroless Plating: Fundamentals and Applications”, NoyesPublications, 1990, which is also included in its entirety by referenceherein.

Using the same ECD tool to reduce the ALD-generated metal oxide filmsinto seed metal films and to form the subsequent metal layers avoids thenecessity of an additional gas-phase reduction module, thereby savingtime and money. ECD tools have small footprints, so using ECD tools forreduction also saves floor space in the clean room for more ALD modulesfor forming the barrier layers and CuO layers in the same metallizationcluster.

Nickel oxide (NiO), silver oxide (AgO), cobalt oxide (CoO) and rutheniumoxide (RuO₂) serve as examples of other metal oxides with which ECD canbe used.

A surprising finding related to certain embodiments described herein isthat the film has very good adhesion to the substrate, even after areduction step. The structural integrity of the metal film is preservedand the formation of pinholes in the film is avoided. While the exactnature of the interface between the metal film and the substrate isunclear, it is obvious that the interface is much stronger than in thecase of direct deposition of metal films by ALD.

ALD-grown metal oxides are denser than PVD- or CVD-grown metal oxides.It would be logical to assume that it is difficult to apply anyelectrochemical reduction or deposition processes on surfaces that havedense structure and poor electrical conductivity. A surprising findingrelated to certain embodiments described herein is that theelectrochemical reduction process could successfully be applied to thereduction of dense ALD-grown metal oxides that are poor electricalconductors before the reduction process.

EXAMPLES Example 1 ALD of Copper Oxide, Method I

The deposition was carried out in an F-200 ALCVD™ reactor manufacturedby ASM Microchemistry Oy, Finland. Cu(thd)₂ was loaded into an externalsource container and heated to 180° C. The flow of the O₃/O₂ mixture wasset to 120 std·cm³/min. There was about 15% of O₃ in O₂. A 200 mmsilicon wafer was loaded through a load lock into the reaction chamberof the reactor. The pressure of the reaction chamber was adjusted toabout 5-10 mbar with a vacuum pump and flowing nitrogen gas. Thereaction chamber was then heated to 210° C. One pulsing cycle consistedof 0.5 s Cu(thd)₂ pulse, 1.0 s N₂ purge, 0.5 s O₃ pulse and 1.0 s N₂purge. The pulsing cycle was repeated 3000 times. The thin film grown onthe wafer had a brownish gray color, high resistivity and excellentadhesion. The thin film consisted of CuO, which was treated with anorganic reducing agent to create a metallic copper metal film on thewafer. The reduced thin film had good conductivity and showed excellentadhesion to the substrate.

Example 2 ALD of Copper Oxide, Method II

The deposition of copper oxide was carried out according to Example 1,with CuCl used as the copper source chemical. Pieces of siliconsubstrates with and without a TiN coating were loaded into the reactionchamber of the reactor. CuCl was loaded into a source tube and thereactor was evacuated. The pressure of the reaction chamber was adjustedto about 3-10 mbar with a vacuum pump and flowing N₂ gas. The reactionchamber was heated to 380° C. and CuCl to about 360° C. Copper oxidegrew in a controlled manner and with excellent adhesion to both of thesubstrates and had the same performance as the CuO film obtained inExample 1.

Example 3 ALD of Copper Oxide, Method III

Surprisingly, it was found that copper oxide (Cu₂O) can be grown by ALDusing anhydrous copper nitrate Cu(NO₃)₂ and an aqueous solution of NH₃as source chemicals. It turned out that dry gaseous ammonia does notform Cu₂O with anhydrous copper nitrate.

Cu(NO₃)₂ was synthesized according to the instructions of C. C. Addisonet al. (J. Chem. Soc. (1958) pp. 3099-3106). Wet NH₃ vapour was formedby evaporating aqueous solution of NH₃ at room temperature from anexternal source bottle. The temperature of the Cu(NO₃)₂ source tube wasset to 120° C. Silicon and TiN-coated silicon were used as substrates.The substrate temperature was about 150° C. Higher substratetemperatures were also tested. However, it was found that it isdifficult to control the film uniformity at higher growth temperatures,possibly because of the thermal decomposition of copper nitrate in gasphase.

One pulsing cycle consisted of four steps, in the following order:Cu(NO₃)₂ pulse (1.0 s), N₂ purge (2.0 s), NH₃*H₂O pulse (2.0 s), N₂purge (2.0 s).

The thin film grown at 150° C. on the substrate had a growth rate of 0.2Å/cycle and according to EDS measurements consisted of Cu₂O.

Example 4 ALD of Copper Oxide, Method IV

The deposition was carried out in an Pulsar 2000 ALCVD™ reactormanufactured by ASM America, Inc., USA. Cu(acac)₂ was loaded into anexternal source container and heated to a temperature that was selectedfrom a range of 130-180° C. The flow rate of the O₃/O₂ mixture wasprogrammed to 200 std·cm³/min (sccm). There was about 15% of O₃ in O₂. A200-mm silicon wafer was loaded through a load lock into the reactionchamber of the reactor. The pressure of the reaction chamber wasadjusted to about 5-10 mbar with a vacuum pump and flowing nitrogen gas.The reaction chamber was then heated to a temperature that was selectedfrom a range of 135-190° C., preferably to 135° C. One pulsing cycleconsisted of 0.5 s Cu(acac)₂ pulse, 1.0 s N₂ purge, 1.0 s O₃ pulse and1.0 s N₂ purge. The pulsing cycle was repeated 1000 times. The thin filmgrown on the wafer had a brownish gray color, high resistivity andexcellent adhesion. The thin film consisted of CuO, which was treatedwith an organic reducing agent ethanol to create a metallic copper metalfilm on the wafer. The reduced thin film had good conductivity andshowed excellent adhesion to the substrate.

Example 5 ALD of Cobalt Oxide

Co(thd)₃ and O₃ were used as source chemicals for the cobalt oxidedeposition. Co(thd)₃ was heated to 110° C. O₃ was prepared from 99.9999%O₂ with an external ozone generator. The resulting oxygen source gasmixture consisted of 10-20 vol.-% O₃ in O₂. Nitrogen, evaporated fromliquid nitrogen, was used as an inert purging gas. Co(thd)₃ pulse lengthvaried from 1.5 s to 2.0 s, while O₃ pulse (flow rate 100 std·cm³/min)length varied from 2.0 s to 4.0 s. Silicon was used as the substratematerial. Substrate temperatures between 150° C. and 350° C. weretested. One pulsing cycle consisted of four sequential steps: Co(thd)₃pulse, N₂ purge, O₃ pulse, N₂ purge.

The higher deposition temperature tested resulted in uncontrolled filmgrowth, as Co(thd)₃ decomposed thermally thus producing a poor thicknessprofile for the thin film. At the lower substrate temperatures, acontrolled growth rate of the thin film (0.3 Å/cycle) and good adhesionwere obtained. A total of 2000 pulsing cycles resulted in a 64 nm thickcobalt oxide layer. According to Energy Dispersive X-ray Spectroscopy(EDS) measurements the thin films consisted of CoO.

Example 6 ALD of Palladium Oxide

Substrates with Si, TiN, WN, W₃C and SiO₂ surfaces were loaded into anF-120 ALCVD™ reactor manufactured by ASM Microchemistry Ltd., Finland.Pd(thd)₃ was loaded to a solid source tube of the reactor. The reactorwas pumped to vacuum. The pressure of the reaction chamber was adjustedto about 5-10 mbar with flowing nitrogen gas while the pumping of thereactor continued. The Pd(thd)₃ was heated to 110° C. and the reactionchamber to 150° C.

One pulsing cycle consisted of four steps in the following order:Pd(thd)₃ pulse (2.0 s), N₂ purge (1.0 s), O₃ pulse (4.0 s), N₂ purge(2.0 s).

The growth rate of palladium oxide from Pd(thd)₃ and O₃ was 0.15 Å/cycleat 150° C. According to EDS the film consisted of palladium oxide. Thefilm grew on Si, TiN, WN, W₃C and SiO₂ surfaces and showed goodadhesion.

Example 7 ALD of Ruthenium Oxide

Ruthenium oxide was grown from alternate pulses ofbis(ethylcyclopentadienyl)ruthenium (EtCp)₂Ru and water. The (EtCp)₂Rusource container was heated to about 90-95° C. Evaporated (EtCp)₂Ru wasintroduced into the reaction chamber, which was heated close to 200° C.,and contacted with the substrate surface for 1.0 s. The reaction chamberwas purged with inert nitrogen gas for 0.5 s to remove residual(EtCp)₂Ru from the gas phase. After that, water vapor, which wasevaporated from an external source bottle, was introduced into thereaction chamber and contacted for 2.0 s with the substrate surface,where H₂O molecules reacted with adsorbed Ru compound molecules. Thenthe reaction chamber was purged with inert nitrogen gas to removeresidual H₂O and reaction by-products. The set of pulses was repeateduntil a ruthenium oxide thin film with the desired thickness was grownon the substrate. Optionally some amount of oxygen gas may be added tothe H₂O flow to control the oxidation state of Ru. Relatively mildoxidizing agents were used. Strong oxidizing agents, e.g. ozone, have atendency to oxidize part of the adsorbed ruthenium into its maximumoxidation state +8 and the resulting RuO₄ is highly volatile and thusthe growth of RuO₂ is disturbed because of desorbing RuO₄.

Example 8 Reduction of Copper Oxide Using Chemical Reducing Agent

Oxidized ECD copper oxide samples covered with around 350 nm of a copperoxide layer were reduced in an ALD reactor. Several organic compoundswere tested as reducing agents. Resistance of non-oxidized ECD coppersamples, oxidized ECD samples and reduced ECD copper oxide samples weremeasured. ECD copper metal samples consisted of pure metal deposited onsilicon substrate.

Nitrogen gas (N₂) of a purity of 99.9999% was used as a carrier gas forthe reducing agents. Tested reducing agent were MeOH (methanol), EtOH(ethanol), 2-PrOH (isopropanol), tert-BuOH (tert-butanol), PrCHO(butyraldehyde), Me₂CO (acetone), HCOOH (formic acid), CH₃COOH (aceticacid), and H₂ (hydrogen, for a comparison).

The effect of different reducing temperatures is shown in FIG. 4, wherethe number after the reducing agent is the reduction temperature indegree C. Also, different reaction times were tested. The reaction timein minutes is given in parenthesis.

In FIG. 4 ECD-1 and ECD-2 stands for pure ECD copper metals without anyadditional treatments. O-ECD-1 and O-ECD-2 indicates samples that have acopper oxide coating (about 350 nm). The resistance values in R (ohm)measured from the samples after the reduction step are averages of 10measurements.

Example 9 Reduction of Copper Oxide Using an Electric Current as aReducing Agent and Integration of the Method to the Preceding andSubsequent Process Steps

FIG. 5 schematically illustrates a first cluster tool 100 in accordancewith embodiments described herein. The substrate is cleaned (e.g.,sputter-cleaned using nitrogen, ammonia, or argon plasma) in the firstreaction chamber 110 of the cluster tool 100. The substrate is thenmoved to the second reaction chamber 120 of the cluster tool 100, inwhich a diffusion barrier layer, for example tungsten nitride carbide(WNC), is deposited by ALD on the substrate. The thickness of WNC can beselected, e.g., from the range of 1-6 nm. Then the substrate is moved tothe third reaction chamber 130 of the cluster tool 100 in which copperoxide (CuO and/or Cu₂O) is deposited by ALD on the diffusion barriersurface (e.g., WNC). The thickness of copper oxide can be selected,e.g., from a range of 0.6-10 nm. The first cluster tool 100 furthercomprises a vacuum transport module 140 and one or more load locks 150to transfer the substrate to and from the cluster tool 100.

A barrier processing sequence is also possible in which the diffusionbarrier is deposited before sputter-cleaning of the vias. The benefit ofthis type of processing sequence is that copper removed from the viabottom during the cleaning step cannot contaminate the via walls becausethe sidewalls are covered with the copper diffusion barrier as the Cu atthe bottom of the via is cleaned by the directional etch.

After copper oxide deposition, a robot moves the substrate to a loadlock and the substrate is exposed to clean room air. The substrate isthen transported to the next processing unit, e.g., a second clustertool 200 or an ECD tool. Use of an ECD tool is preferable because suchECD tools are typically less costly than a CVD bulk copper cluster tool.

FIG. 6 schematically illustrates a second cluster tool 200 in accordancewith embodiments described herein. The copper oxide film on thesubstrate is reduced into copper metal with a wet electrochemicalprocess in the first module 210 of the second cluster tool 200. Thesubstrate is placed to a solution that contains sodium hydroxide orother water-soluble metal hydroxide. The solution is preferably kept at+50° C.-+60° C. and inert gas (e.g., nitrogen) is bubbled through thesolution to remove free oxygen gas from the solution. A bias voltage of1.4 V is applied between the substrate and the opposing electrode. As aresult, deposited copper oxide on the substrate is reduced into coppermetal and oxygen gas forms on the opposing electrode. In otherembodiments, copper oxide is reduced into copper metal by using hydrogenplasma or alcohol, aldehyde, or carboxylic acid. The copper surface canalso be exposed to an iodine compound (e.g., ethyl iodide) to activatethe copper metal surface. The copper metal film is used as a seed layerfor subsequent bulk copper deposition using the electroplating processto fill vias and trenches.

After rinsing the substrate to remove residual solution, the substrateis moved to the second module 220 of the second cluster tool 200. Thecopper seed layer is protected against re-oxidation during the transportfrom one module to another with an inert gas atmosphere in the vacuumtransport module 230 that optionally may contain some reducing gas suchas hydrogen, alcohol, or aldehyde. In the second module 220, thesubstrate is placed in an electroplating solution that contains awater-soluble copper compound, some acid to lower the pH of thesolution, and standard additives that are commonly used to improve thequality of the growing copper film. A voltage is applied between thesubstrate and an opposing electrode. Copper is deposited from thesolution on the seed layer, and vias and trenches become filled withcopper metal. The opposing electrode consists of pure copper thatdissolves to the electroplating solution during the electroplatingprocess. After the electroplating process, the substrate is rinsed toremove residual electroplating solution. The substrate is then ready forchemical mechanical polishing (CMP). The second cluster tool 200 furthercomprises one or more load locks 240.

According to still another embodiment of the present invention metaloxide is first reduced with current in a bath into seed layer and thenelectroless plating is used for depositing metal on the seed layer. Incase of electroless copper plating, the bath can for example consists ofwater-soluble copper compound (e.g. 12-16 g of copper sulfate/liter),reducing agent (e.g. 10-12 g of formaldehyde/liter), chelating agent(e.g. 18-21 g of ethylene diamine tetraacetic acid EDTA/liter),complexant (e.g. 10-12 g of sodium potassium tartrate/liter), stabilizer(e.g. 2-mercaptobenzothiazole) and alkali compound (e.g. 13-17 g ofNaOH/liter) for raising pH. Copper oxides do not dissolve in alkalinesolutions, so it is possible to use the same bath for reducing metaloxide with current and for electroless plating. Thus, only one processmodule is needed for bulk copper fill.

Various embodiments of the present invention have been described above.Although this invention has been described with reference to thesespecific embodiments, the descriptions are intended to be illustrativeof the invention and are not intended to be limiting. Variousmodifications and applications may occur to those skilled in the artwithout departing from the true spirit and scope of the invention asdefined in the appended claims.

We claim:
 1. A method of depositing ruthenium oxide on a substrate in areaction chamber by an atomic layer deposition process comprising:providing a pulse of a ruthenium source into the reaction chamber;purging the reaction chamber of excess ruthenium source; providing apulse of an oxygen source into the reaction chamber; and purging thereaction chamber of excess oxygen source, the method additionallycomprising after purging the reaction chamber of excess rutheniumcompound and prior to providing a pulse of an oxygen source: providing apulse of a chemical reducing agent into the reaction chamber; andpurging the reaction chamber of excess reducing agent, wherein thechemical reducing agent chemically reduces the ruthenium source.
 2. Themethod of claim 1, wherein the ruthenium oxide is selected from RuO₂ andRuO₄.
 3. The method of claim 2, wherein the ruthenium oxide comprises aternary ruthenium compound.
 4. The method of claim 2, wherein theruthenium oxide has a resistivity of about 35 μΩ-cm.
 5. The method ofclaim 1, wherein the ruthenium source is a ruthenium halide.
 6. Themethod of claim 5, wherein the ruthenium halide is chosen from a groupconsisting of RuF₅ and RuF₆.
 7. The method of claim 1, wherein thereducing agent comprises at least one of alcohols, aldehydes, carboxylicacids, and hydrogen radicals.
 8. The method of claim 1, wherein theoxygen source is chosen from a group consisting of water (H₂O), hydrogenperoxide (H₂O₂), ozone (O₃), oxygen (O₂), singlet oxygen ¹O₂, oxygencompounds with unpaired electrons, and oxygen and OH radicals.
 9. Amethod of depositing a thin film comprising ruthenium oxide on asubstrate by a plurality of atomic layer deposition cycles, each cyclecomprising contacting the substrate with alternating vapor phase pulsesof a Ru compound and an oxygen compound, wherein each cycle additionallycomprises contacting the substrate with a reducing agent that chemicallyreduces the Ru compound.
 10. The method of claim 9, wherein theruthenium oxide is selected from RuO₂ and RuO₄.
 11. The method of claim9, wherein the ruthenium oxide comprises a ternary ruthenium compound.12. The method of claim 11, wherein the thin film additionally comprisesstrontium.
 13. The method of claim 12, wherein the substrate isalternately contacted with the ruthenium compound, the oxygen compoundand a strontium compound.
 14. The method of claim 13, wherein the filmcomprises SrRuO₃.
 15. The method of claim 9, wherein the rutheniumcompound is a ruthenium halide.
 16. The method of claim 15, wherein theruthenium halide is chosen from a group consisting of RuF₅ and RuF₆. 17.The method of claim 9, wherein the oxygen compound is selected from agroup consisting of water (H₂O), hydrogen peroxide (H₂O₂), ozone (O₃),oxygen (O₂), singlet oxygen (¹O₂), oxygen compounds with unpairedelectrons, and OH radicals.
 18. The method of claim 9 comprising atleast three deposition cycles.
 19. The method of claim 9, wherein thereducing agent is an organic reducing agent.
 20. The method of claim 9,wherein the thin film is used as an electrode.
 21. The method of claim20, wherein the electrode is the gate electrode of a transistor.
 22. Themethod of claim 20, wherein the electrode is part of a capacitor.